在JPA 2中使用投影

这是一个老问题，但让我们举个例子：

使用

CriteriaBuilder

CriteriaBuilder cb = em.getCriteriaBuilder();//We want Integer resultCriteriaQuery<Integer> cq = cb.createQuery(Integer.class);//The root does not need to match the type of the result of the query!Root<Islem> isr = cq.from(Islem.class);//Create a join to access the variable status of working dayJoin<Islem,WorkingDay> join = isr.join("workingDay",JoinType.INNER);//Create the sum expressionexpression<Integer> sum = cb.sum(isr.get("amount"));cq.where(         cb.equal(isr.get("currency"), CURRENCY),         cb.notEqual(isr.get("status"), INACTIVE),         cb.equal(isr.get("product"), product),         cb.equal(join.get("status"), ACTIVE)).select(sum);

另一方面，如果要查询实际的“金额”值，则可以执行以下操作：

CompoundSelection<Integer> projection = cb.construct(Integer.class, cb.sum(isr.get("amount")));cq.where(..).select(projection);List<Integer> amounts = em.createQuery(cq).getResultList();